I often do 4*4 intervals while running, and stay at around 85-90% of my maximum heart rate (in the later intervals I reach this quickly). It feels right, and the lactic acid build-up doesn't hinder my next set of intervals. I've heard some people claim you should run at 95-100% during the last set, but others think acid build-up only is detrimental. What do you think? What would be the pros and cons of hitting 100% versus "only" 90% in the last set? I think my experience with strength training has been that you don't progress as fast unless you feel a lot of acid build-up during your last rep.

Answer: It is not damaging to run at 95-100% in the last interval, but it can be discouraging, making it harder to keep exercising in the long run. The benefit is practically negligible compared to staying at 90%.

How exhausted should you be?

If I run four intervals, should I be completely exhausted after every interval and rather take a longer break between intervals, or is it better to not tire myself out during the first couple intervals, and only take a two to three minute break?

Answer: You should feel capable of doing another interval after you've completed the four – if this seems impossible, you're too exhausted. For the sake of maintaining motivation, you should be equally tired after very interval – if you're going to exhaust yourself, at least wait until the last rep. Otherwise, most people get such a high lactic acid build-up that they are unable to reach optimal training intensity during the remaining reps.

Only for "healthy" people?

About 7-8 years ago I had a stent inserted in a coronary artery after a small infarction. Therefore, I'm a little reluctant to start exercising. Is this method appropriate for people like me? How would I start training? I feel like little attention and inadequate information has been provided to people like me.

Answer: Your experience is common and it's a shame the health authorities don't have a better system in place for people like you. We have the same experience with people approaching us as you describe – they are concerned when they come to us, but see that exercise isn't dangerous after we guide them through a training session. Many people falsely associate interval training with running until you drop, but this is definitely not the case. Our experience with exercise and people like you has been positive, but I don't know your exact background, so if you're unsure you should contact your doctor get a referral to a hospital where you can do an exercise test. That way, you'll get your answer from very competent people.

The right heart rate?

Hi! I absolutely love 4*4 intervals, but am confused about heart rates. I often read or hear that you shouldn't exceed 70-75% of your maximum heart rate, even during interval training. Simply put, I find it really difficult to avoid, since I reach 80% really quickly, so if I'm supposed to stay around 70%, I practically have to walk, which I don't find particularly strenuous :) Yesterday, when I had finished 4*4, my average heart rate was 90%, and no lactic acid build-up in sight. Is this normal? I really prefer running my intervals at 90-95% of maximum heart rate. Will this help me progress? Am I getting into better shape?

Answer: Never exceeding 70-75% is stupid advice. If you're keeping your intensity that low, there's no need for intervals – you can probably keep it up for hours. And if you had no acid build-up at 90%, that's probably not actually 90% … unless you're in great shape, that is. If you've used 220 minus your age to calculate your maximum heart rate, that's terribly inaccurate for most people – sometimes off by as much as 40bpm. I recommend that you actually test your maximum heart rate; warm up thoroughly to get you sweating, then 2 interval-sets of 4 minutes each, so you're short of breath. After 2 minutes on the third set, give everything until you reach exhaustion… and that's your maximum heart rate – at least a pretty close estimate. Good luck!

Heart rate in between 4x4 intervals?

Hi! During 4 min intervals I have no problem staying at around 180-190 bpm (I'm 33). Between reps I try to calm down, but even when jogging very slowly I find it hard to get under 130-140 before the next 4 min run. Should I walk between intervals to get my heart rate down? And how low should it ideally go?

Answer: It sounds like what you're doing is perfect – I estimate your maximum heart rate to be roughly 200 if you're at around 170-180 during interval training. That makes 140bpm just right during breaks, since you remove lactate most efficiently at 70% of your maximum heart rate. Keep up the good work!

Heart flutter and interval training?

Hi! A couple years ago, I experienced a heart flutter, and since then, I haven't dared to push myself to high heart rates out of fear that my heart will start to flutter. Are my fears unwarranted? Can people with heart flutters exercise heavily, or should they keep their heart rates low?

Answer: That's a good question, and currently there is no clear answer. I presume it was an atrial flutter, which is uncomfortable, but not normally dangerous. Jostein Grimso from Feiringklinikken has shown that after being active for many years, endurance athletes have an increased occurrence of atrial flutters – however this is coupled with an increase in longevity. We actually don't know how it goes with people with atrial flutters without many years of extreme exercise behind them, so we've started a study to find out. It seems like the incidence of atrial flutters decreases in this group when they exercise. However, we won't know for sure for another 2 years, when the study is completed. I would talk to your doctor, and possibly get a referral to a work-ECG at a hospital to find out what you should do now.

Level ground or incline?

I feel like I can control my workout better if I do it on a treadmill and can program inclines and speed. Recently I've only been running outdoors. Should I be running uphill? When is the right time to reach the desired intensity – should I be at 90% after 20 sec or is it enough if I reach it by the last two minutes?

Answer: It's easier not to "cheat" on a treadmill since you've programmed the speed and slope. Reaching a high pulse uphill is easier since you mobilize more muscles (unless you're a very good runner) and it prevents damage to muscles and joints. A lot of people wonder about intensity. We're content as long as you reach 90% of your maximum heart rate during the first 4 min interval. However, for reps 2-4 it shouldn't take you more than 2 minutes to reach the right intensity.

How to define 90% of maximum heart rate?

Perhaps you can help me with the definition of heart rate as it pertains to 4x4. When I do 4x4, my trainer tells me to stay at roughly 70% between reps, and 85-95% during them. However, it seems like people define the "zero" compared to maximum heart rate differently. Some people say that zero is your resting heart rate, while others think zero intensity means a heart rate of 0 bpm. At high intensity, this makes little to no difference, but at 70%, it can really matter quite a lot. Which is correct? Personally, I feel using the resting heart rate makes most sense, because that seems like zero effort to me.

Answer: It is correct that one ideally should be at around 70% of maximum heart rate during breaks – the reason for this being that this is where people remove lactate most efficiently. As you note, there are many ways to define exercise intensity. We want to make it simple so define the highest heart rate measured = maximum heart rate. If you start including the resting heart rate, it becomes too complicated for most people. Besides, by this definition, there's no need for a heart rate monitor – the only requirement is doing 4 minutes and being somewhat short of breath without experiencing extensive muscles stiffness (but a little), and staying active during the break (no standing around). If you're using maximum heart rate minus resting heart rate, the scale becomes something rather different, however some publications show both. Hopefully that answered your question!

Does 4x4 interval training burn fat?

Answer: A recurring question has been whether you should run or walk in order to burn as much fat as possible. Put differently, what is better – high- or low-intensity exercise? The background for this question is that during low-intensity muscular work (30-40% of maximum oxygen uptake, a measure of physical fitness), the metabolism of fat comprises roughly half the total energy, while only 20-30% of the energy (or less) comes from fat during high-intensity physical activity like interval training. While 55% of the energy comes from fat at 40 % of maximum oxygen uptake, this is reduced to 20% at an intensity of 80%. This knowledge has led to recommendations that people desiring to burn as much fat as possible should do low-intensity exercise – a message amplified by the use of terms like "fat burning exercise."

However, claims like these are based on a misunderstanding. In reality, it is the total energy spent, as opposed to the percentage of energy from fat that should be the concern. Under high-intensity exercise, not only is the energy expenditure per unit of time greater, but totally, more fat is metabolized compared to low-intensity exercise, even if the percentage of energy from carbohydrate is greater.

For instance, at 75% of maximum oxygen uptake, the energy from fat and carbohydrates can be, respectively, approximately 30 and 70%, while at 40% of maximum oxygen uptake (so-called "fat burning exercise"), they might be roughly 50-50. If in the first case the energy expenditure were 12kcal/min, and in the second it were 6kcal/min, in the course of an hour one would use 720kcal and 360kcal, respectively. Of these, respectively 216kcal and 180kcal would be from fat. This shows that during high-intensity exercise, not only is more energy spent, but more fat is also burned per unit of time. Similarly, you don't lose more fat if you bike a given distance at a relaxed rate versus biking the same distance at a higher speed in a shorter amount of time.

Daily exercise?

How many times a week do I have to work out in order to improve my current "shape" (currently at approx. 30 min every day)?

Answer: Half an hour, seven days a week is great, but it's important to note a significant health benefit with less too. Additionally, I think it's overdue that high-intensity physical activity were included in official Norwegian recommendations. In the U.S., the recommendation has long been 20 minutes of high-intensity exercise, three times a week as an alternative to more frequent and longer moderate-intensity workouts. Past studies have also shown that if the intensity of an activity is high, one to two workouts a week can have a significant health benefit.

Is it dangerous for pregnant women to do 4x4 interval training? Is there scientific evidence of a maximum pulse one should reach, after which there is damage to the foetus?

Answer: According to international guidelines, exercise with moderate intensity is safe pregnancy. As of this time, not much research has been done on interval training or high-intensity exercise during pregnancy, but it looks like pregnant women are fine exercising up to 90% of maximum heart rate.

I have a question pertaining to your 4x4 interval training program (4x4 intervals with 85-95% of maximum heart rate with 3 min. "breaks" at 60-70%). I exercise according to your regimen, and my question is – how often would you recommend that I do interval training? Can one do it an unlimited number of times in a given amount of time and still get the health benefits, or is the strain on the heart so great that one should limit the frequency? How many times a week would you recommend?

Otherwise, I'd like to thank you for an inspirational program where I can use the heart rate monitor to track my progress.

Answer: It's great that you feel inspired by our program! It's a very efficient and fun way to exercise. Generally speaking, it looks like 1 set of intervals a week is enough to maintain your current fitness, and a greater frequency is required to improve one's physical fitness. Nevertheless, it bears noting that individual effects may vary, and it also depends on your overall fitness.

If the goal is to increase your physical fitness, we recommend 2-3 sets of interval training per week. We don't know the optimal number of training sessions, and it may vary from person to person. In one of our studies, test subjects did 5-8 sets of intervals every week, but they found it tough to keep up motivation. Moreover, we found that the people doing intervals around 8 times a week were overspent, and didn't get the desired physical adaptations, like an increased oxygen uptake.

Losing weight by interval training?

Hi! I saw a program on TV2 about teenagers losing weight in a research study from the Faculty of Medicine and Health Sciences at NTNU. They had done interval training. What sort of regimen were they on?

Answer: We included very obese teenagers who were referred to St. Olav's Hospital due to their obesity. The test subjects were randomly placed in a physical fitness group or a multi-departmental group.

The physical capacities of children, teenagers and adults vary greatly, with some of the factors involved being age, fitness level and body weight. It is therefore necessary to adapt the exercise intensity to every individual's physical capability, measured as a given percentage of that person's peak oxygen uptake (VO2max). Our research has shown that the best effect is seen by increasing the intensity over time as fitness improves. Interval training with an intensity of 85-95% of the maximum heart rate appears to give the optimal fitness effect.

Once we had determined the teenagers' maximum capacities, we determined the corresponding heart rates for an optimal exercise effect. Following that, the teenagers exercised on treadmills twice a week for 12 weeks under professional supervision while wearing heart rate monitors to ensure that the intensity was correct.

The exercise program comprised of a 10-minute warm-up at 60-70% of maximum heart rate followed by 4x4 intervals at 85-95%, with 3-minute active breaks between each interval (at 60-70%), finally ending with a 5-minute cool-down at 60-70% of maximum heart rate.

After these 12 weeks of exercise, we let the teenagers take more personal responsibility, exercising at home or at a gym. They were advised to keep their workout routines the same – interval training 2-3 times per week. They returned for check-ups 1-2 times a month for 1 year.

The program was all about exercise – the only information we provided about dietary choices was a single 30-minute lecture on nutrition given at the onset of the research project.